We don't need to roll back tree-breaking changes because the bots are the only ones allowed to land changes in the first place, and they make sure all tests pass before they land. There's no reason a bot couldn't do this, though.
That said, Google might consider it easier to work with Gerrit, Rietveld, Gitiles, etc. because they can work with the source code of these projects directly, than having to write potentially ugly code to work around missing pieces in GitHub's API.
I'd love to learn whether a large scale project such as Rust has run into these issues with GitHub.
There's some stuff that could be nicer with GitHub generally, but I don't think having the source would make it any easier to build out this automation; github's API is pretty solid.
The automation would be even more important for a codebase like Chromium that receives 5-8 change lists every minute [2].
[1] https://github.com/rietveld-codereview/rietveld
[2] https://codereview.chromium.org/